This invention was made with United States Government support under a contract with the United States Navy. The United States Government has certain rights to this invention.
The present invention relates to dynamoelectric machines and, more particularly to the reduction of harmonic losses in alternating current (ac) dynamoelectric machines.
Dynamoelectric machines may be either motors or generators. In order to be specific, and by way of example, the present invention is described as applied to a round rotor ac synchronous motor. However, the following description and the present invention are equally applicable to a round rotor ac synchronous generator as well as other types of rotating electric machines.
A conventional round rotor ac synchronous motor commonly utilizes a solid magnetic steel forged rotor. The field windings are inserted into longitudinal slots which extend axially along the rotor and are provided with a direct current (dc) energizing current to produce magnetic poles and a magnetic field in the rotor. The rotor is rotatably mounted within the stator which is typically formed from a stack of laminated magnetic metal sheets which are insulated from one another. The stator includes windings energized by ac currents which produce a rotating magnetic field. The interaction of the rotating magnetic field in the stator with the magnetic field produced in the rotor causes the rotation of the rotor in a manner well known in the art.
Some types of ac motors control the motor from a dc source, or from an ac source having a frequency which is different from the frequency required by the motor for the desired speed, or vary the frequency supplied to the motor in order to control the speed of the motor while driving the motor from a constant frequency ac source. Common methods of providing the required input power to such motors include the use of a load commutated inverter or a cycloconverter. One form of current source inverter and one form of cycloconverter suitable for use with the present invention are described in U.S. Pat. No. 4,843,271 issued Jun. 27, 1989 to M. R. Shah, assigned to the same assignee as the present invention, and hereby incorporated by reference. That patent points out that the use of a current source inverter or a cycloconverter produce harmonics having relatively high frequencies which in turn produce eddy current in the surface of the rotor of the motor. The eddy current consumes power and reduces the efficiency of the motor, while at the same time providing unwanted heating of the motor. The power loss and heating are significant because of the relatively high electrical resistivity of the rotor steel. Public concern over pollution caused by the generation of electric power, and over the consumption of energy resources in the generation of electric power, has provided an increased and continuing demand for higher efficiency electric motors and generators. In the aforesaid U.S. Pat. No. 4,843,271 these undesirable characteristics are decreased by a coating of a high-conductivity metal covering a portion of over 10% of the surface of the rotor, and in one embodiment surfaces of dove-tail inserts which retain the rotor conductors in the longitudinal slots are coated with copper. The present invention constitutes an improvement over the aforesaid U.S. Pat. No. 4,843,271 as described below.